Today, signals are broadcast in many ways. Signals can be broadcast over cables, by laser, in the atmosphere, in space, under water and through other media. These signals contain information which is transmitted in varying forms. One common form of signal transmission is frequency modulation (FM).
Both amplitude and frequency modulation are methods of superimposing information onto a carrier signal. Specifically, in FM broadcast, a carrier wave is used to transmit information to a receiving audience. This is most commonly known as FM radio. Consequently, the frequency of the carrier signal is caused to vary in accordance with a voltage variation (message) to be carried.
In an FM transmission, a sinusoidal carrier signal having variations of instantaneous frequency is received. These variations can be either above or below the center frequency of the carrier signal. Thus, a detecting device (demodulator) for detecting these variations is typically constructed such that its output will vary according to the instantaneous frequency of an incoming signal. Such detecting devices are sensitive to variations produced by interfering signals (interference) or by non-linearities in components of the receiver. As these detecting devices have frequency dependent components, the above-mentioned non-linearities and interference affect signal demodulation. Practical FM detectors known in the prior art include slope detectors (balanced discriminators, ratio detectors and quadrature demodulators), differentiators, pulse-counting discriminators and phase-locked loops.
In U.S. Pat. No. 4,859,958, by the inventor of the present invention, a demodulator which provides improved demodulation of all of several FM carriers, including weaker signals in the presence of dominant carriers, is disclosed. In that patent, a frequency demodulator converts the instantaneous frequency of the applied signal to a voltage. When the sum of two or more signals is present at the input to the demodulator, the output voltage is proportional to the instantaneous frequency of the dominant portion of the input signal due to the capture associated with FM demodulations.
Prior art FM detectors are often characterized as exhibiting "threshold effect." The quality of the recovered message is commonly indicated by the value of its signal-to-noise ratio (SNR). The value of the SNR of the recovered message is related to the value of the SNR of the input (radio-frequency voltage) to the receiver. For good quality reception (large values of SNR of the receiver input), the message quality is linearly related to the quality of the receiver input. Threshold of the receiver is defined as that value of receiver input SNR (signal quality) where the aforementioned relation ceases to be linear. For prior art FM detectors, there is a marked departure from linearity for values of receiver input SNR less than the threshold value. Therefore, the onset of "threshold effect" is a sudden loss of quality of the recovered message. When the quality (SNR) of the received signal is "below threshold," the quality of the recovered message deteriorates rapidly with decreasing values of input SNR.
The phenomenon of threshold effect typically causes a loss of message quality when the quality of the received carrier is in the range from 8 to 13 decibels for prior art FM detectors. Anyone who has ever been driving and listening to music on their FM radio when suddenly the music intermittently cuts out and is replaced with static-like noise, has experienced threshold effect. Thus, there is no `graceful degradation` of FM receivers operating "below threshold."
Prior to the present invention, threshold effect was generally believed to be caused by the nature of FM signal transmission. A common perception is that `strong` interference creates "spikes" or "impulses" of noise at the receiver output which result in a crackling sound. Lathi, B. P., Modern Digital and Analog Communications Systems, Holt, Rinehart and Winston, Inc. (Florida, 2nd ed. 1989), pp. 490-492. The perception is caused by analysis based on the concept of instantaneous frequency of the carrier. The present invention recognizes that frequency is a defined quantity and not a physical or natural property of the FM carrier (receiver input).
The perception that "threshold effect" is an inherent part of FM is not supported in practice because different demodulator circuits have different values of threshold. It is well known that phase-locked loop demodulators have superior noise performance (smallest values of threshold) and pulse-counting discriminators have inferior noise performance (largest value of threshold) in the family of prior art FM detectors. These differences in values of threshold support a conclusion that "threshold" is caused by the detector circuitry and not by the nature of the carrier modulation.
According to the teachings and disclosures of the present invention, FM threshold effect is not due to the nature of FM transmission, but rather due to the inability of prior art demodulators to resolve or demodulate source signals (hereinafter source, message, modulating and applied signal(s) are used interchangeably) from carrier signals in the presence of strong interference.
Therefore, a demodulator which can resolve source signals from a carrier wave even in the presence of large amounts of noise is desirable. Additionally, an FM demodulator which can resolve signals such that any degradation therein remains substantially linear for smaller values of input signal-to-noise ratio, i.e., reduction of the value of threshold associated with prior art demodulators, is desirable.
Heretofore, no prior art demodulator uses the measured time intervals between zero values of an FM carrier to detect the source signal and to mitigate the effects of interference.